The operator's station of a vehicle or machine having a hydrostatic transmission has a manually actuated lever. This lever is connected to a manual displacement control (MDC) input handle by mechanical means such as a push/pull cable or linkage means. If this mechanical means becomes disconnected, it is important that the MDC automatically cause the pump in the hydrostatic transmission to return to neutral to stop the vehicle or other load that is driven by the pump.
The MDC typically will have a neutral start switch (NSS), but not a backup alarm switch (BAS). The NSS is sensitive to case pressure. When case pressure is high, such as when the oil is cold, the pressure on the pin can push the pin and actuate the switch. A higher spring force can be used in the switch to counteract high case pressure but this creates a higher friction, and worsens the "stick/slip" characteristic caused by pushing the pin out of the cam detent. These higher forces are undesirable. When a machine operator is trying to very carefully move the input handle (such as inching a vehicle up to an attachment) he will have difficulty if a large force is required to lift the pin out of the detent.
Further, existing MDC's are difficult to adjust on the production assembly line and in the field. Some manual displacement controls are spring centered to neutral via springs on a valve spool in the control. However, many tolerances exist in the linkage between the valve spool and the input shaft. Thus, the switch mechanism must be adjusted to ensure the switch will be open before the valve spool begins to port and put the pump into stroke, but yet provide some reasonable angular range within which the switch is closed. Several complex adjustments are normally required to set the switch to actuate at the correct positions of the input handle. The typical adjustments are external in nature, which makes it difficult to seal the switch and assembly from the external environment. Moisture can seep by the threaded adjustments and corrode the switch to impair its proper functioning.
It is therefore a principal object of this invention to provide a manual displacement control for a hydrostatic transmission that can be removed from the pump of the transmission without having to readjust the neutral start switch assembly.
A further object of this invention is to provide a neutral start switch assembly on a manual displacement control for a hydrostatic transmission wherein a switch can be replaced without having to readjust the neutral switch assembly.
A still further object of this invention is to provide a manual displacement control for a hydrostatic transmission wherein internal hydraulic "case" pressure will not activate the electrical switches of the control.
A still further object of this invention is to provide a manual displacement control for a hydrostatic transmission having a NSS and BAS that can be operated with a single cam.
A still further object of this invention is to provide a manual displacement control for a hydrostatic transmission which is easy to assemble.
These and other objects will be apparent to those skilled in the art.